Revert "readme: image cannot be pulled from the docker hub anymore."
Revert "readme: image cannot be pulled from the docker hub anymore."

This reverts commit 7eb6c8b86dde09fdcf1be5cc76827404ee495144.

The good folks at Wowza Media Systems are ok with making the image
available on the Docker Hub :+1:

waja/docker-wowza,sameersbn/docker-wowza,bruse/docker-wowza,songpr/docker-wowza